Zack,
Right .. you'll have to know what frame geometry will best work for you. I do see, upon occasion, where an internet buyer has to turn around and resell something because it just didn't give the right fit ... even after tweeking saddle / post / bar / stem.
I think as long as you are attentive to the details of actual measurements, how measured (ctr. to ctr. / ctr. to top etc.), your chances of a successful transaction should be fairly decent.
Yes, the opportunity to test ride is always the best option. That DBR I purchased used (locally) I was able to test ride and was amazed it worked out. The frame size is only 15 1/2 ctr./ctr. but the top tube is 22" so I have a lot of titanium seat post exposed which gives me a little 'rear suspension'. If I would have seen those measurements posted on a web-site I would have never considered the bike. The Merlin (e-bay) is an 18.75" ctr./ctr. with a 23.5" tt and both bikes fit me fine with a minimal difference in stem length or rise .. go figure ? ?
My best to you !